The more you use the library, the more you'd want to make a custom shape. And there is a solution for that. However, that talks about manually drawing shapes using the 2d canvas primitives. But, you'd also want to re-use what is already offered by the library. Konva docs currently don't have a say about how to do this.

I may have found a hack. It may not be elegant when you think it. But it will do...